title: Repeated positron emission tomography-computed tomography and perfusion-computed tomography imaging in rectal cancer: fluorodeoxyglucose uptake corresponds with tumor perfusion.
abstract: OBJECTIVE
  The purpose of this study was to analyze both the intratumoral fluorodeoxyglucose (FDG) uptake and perfusion within rectal tumors before and after hypofractionated radiotherapy.
  
  
  METHODS
  Rectal cancer patients, referred for preoperative hypofractionated radiotherapy (RT), underwent FDG-positron emission tomography (PET)-computed tomography (CT) and perfusion-CT (pCT) imaging before the start of hypofractionated RT and at the day of the last RT fraction. The pCT-images were analyzed using the extended Kety model, quantifying tumor perfusion with the pharmacokinetic parameters K(trans), v(e), and v(p). The mean and maximum FDG uptake based on the standardized uptake value (SUV) and transfer constant (K(trans)) within the tumor were correlated. Also, the tumor was subdivided into eight subregions and for each subregion the mean and maximum SUVs and K(trans) values were assessed and correlated. Furthermore, the mean FDG uptake in voxels presenting with the lowest 25% of perfusion was compared with the FDG uptake in the voxels with the 25% highest perfusion.
  
  
  RESULTS
  The mean and maximum K(trans) values were positively correlated with the corresponding SUVs (ρ = 0.596, p = 0.001 and ρ = 0.779, p < 0.001). Also, positive correlations were found for K(trans) values and SUVs within the subregions (mean, ρ = 0.413, p < 0.001; and max, ρ = 0.540, p < 0.001). The mean FDG uptake in the 25% highest-perfused tumor regions was significantly higher compared with the 25% lowest-perfused regions (10.6% ± 5.1%, p = 0.017). During hypofractionated radiotherapy, stable mean (p = 0.379) and maximum (p = 0.280) FDG uptake levels were found, whereas the mean (p = 0.040) and maximum (p = 0.003) K(trans) values were found to significantly increase.
  
  
  CONCLUSIONS
  Highly perfused rectal tumors presented with higher FDG-uptake levels compared with relatively low perfused tumors. Also, intratumor regions with a high FDG uptake demonstrated with higher levels of perfusion than regions with a relatively low FDG-uptake. Early after hypofractionated RT, stable FDG uptake levels were found, whereas tumor perfusion was found to significantly increase.
authors.affiliation: Department of Radiation Oncology, Maastricht University Medical Center, Maastricht, The Netherlands. marco.janssen@maastro.nl
authors.forename: Marco H M
authors.initials: MH
authors.lastname: Janssen
authors.forename: Hugo J W L
authors.initials: HJ
authors.lastname: Aerts
authors.forename: Jeroen
authors.initials: J
authors.lastname: Buijsen
authors.forename: Philippe
authors.initials: P
authors.lastname: Lambin
authors.forename: Guido
authors.initials: G
authors.lastname: Lammering
authors.forename: Michel C
authors.initials: MC
authors.lastname: Öllers
mesh_terms: D000368:Aged; D019583:Dose Fractionation, Radiation; D019788:Fluorodeoxyglucose F18 / Q000493:pharmacokinetics*; D006801:Humans; D064847:Multimodal Imaging / Q000379:methods*; D009367:Neoplasm Staging; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*; D012004:Rectal Neoplasms / Q000098:blood supply* / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000473:pathology / Q000532:radiotherapy; D014057:Tomography, X-Ray Computed / Q000379:methods*
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18